a) Field of the Invention
The present invention relates to a detector for detecting the position of the rotor of a brushless motor.
b) Description of the Related Art
An existing drum driving motor (brushless motor) of a VTR (Video Tape Recorder), for instance, has a rotational position detector for generating a rotational position signal of one pulse per turn of the motor in order to change image heads.
FIG. 6 shows an example of the rotational position detector in which a rotational position detecting step of a motor is formed by the fact that a magnetic field generated by a PG (Pulse Generator) magnet 21a set on the outer periphery of a rotor 21 of the motor is detected by a PG sensor 22 arranged at a position facing the outer periphery of the rotor 21, a signal provided as an output from PG sensor 22 is amplified by an amplifier 23 to obtain a waveform having a peak shown in FIG. 7(a), and the signal is compared with a threshold V.sub.th by a voltage comparison circuit 24 to obtain a pulse signal whose one cycle corresponds to one turn of rotor 21 as shown in FIG. 7(b).
However, the rotational position detector of the above brushless motor has the following problems.
That is, the number of parts increases and therefore the cost increases because the rotational position detector of the brushless motor requires the above-mentioned PG magnet 21a and the PG sensor 22.
Moreover, when a magnetic detector such as PG sensor 22 is used, electric wiring is necessary on a board (printed circuit board) and the operation is complicated. Further, the board cannot be downsized because the magnetic detector is arranged at a position facing the outer periphery of the rotor magnet 21 and a large space for electric wiring is necessary on the board.